Various types of plastic and paper bags are widely used for packaging dry granular or powdery materials. A type of multi-layer paper valve bag, usually with one plastic layer, includes folded and glued ends that permit the bag to assume a rectangular configuration when it is filled with product. One end of the bag is folded and glued to form a valve with a folded flap through which the granular or powdery product is easily poured or otherwise inserted to fill the bag. This opening is designed to self-close when the product in the bag forces the folded flap against the glued top of the bag. Examples of this type of bag are shown and described in U.S. Pat. No. 4,132,347 to Saito and U.S. Pat. No. 4,026,460 to May, the disclosures of which are incorporated by reference.
While paper bags with folded and glued valves are accepted and readily used in various markets, these bags suffer from a variety of problems. When a bag is filled, the product often leaks out of the valve, especially if it is powdery in nature. Paper bags also lose their strength when they are wet. These bags also do not stop moisture entry unless they have an internal plastic liner, which is necessary for many products. The equipment for manufacturing plastic lined, paper valve bags is quite expensive because of the need to fold and glue the paper, and add the internal plastic liner.
Plastic bags are commonly used when strength and moisture entry are a concern. A relatively wide web of a continuous plastic film is processed to form plastic bags and other plastic items by passing the web through a machine that forms a unique combination of folds and thermal seals into the web. The plastic film forming the web can include a number of layers with different melting temperatures. Examples of such plastic bags are shown in U.S. Pat. No. 6,357,915 to Anderson, U.S. Pat. No. 4,524,460 to Twiehoff, U.S. Pat. No. 4,470,152 to Blankenship, U.S. Pat. No. 4,441,209 to Lunshof and U.S. Pat. No. 4,071,187 to La Fleur, the disclosures of which are incorporated by reference. Sections of the web are commonly sealed to each other by passing them through a pair of heat-sealing members while the web moves in a stepped sequence. During a short seal period or dwell time, the web sections are clamped between sealing members to melt and thermal bond or join the sections together and form a seal between those sections. A typical sealer includes sealing members that span the entire width of the machine as shown and described in U.S. Pat. No. 6,422,986 to Claybaker and U.S. Pat. No. 4,019,947 to Stock, the disclosure of which is incorporated by reference. Conventional sealers often include sealing members such as resistance-heated seal wires and a nonstick cloth that covers the seal wires. The resistance-heated wire is generally a nickel-chromium or similar material. The nonstick cloth prevents the plastic web from sticking or otherwise attaching to the surfaces of the sealing members. The cloth is typically treated with a TEFLON coating or similar non-stick material. Conventional sealers also include a servomotor or a pneumatic cylinder that rotates or oscillates a main camshaft to raise and lower a seal bar in repetitive cycles as in U.S. Pat. No. 6,422,986.
A problem with conventional sealing apparatus is their inability to seal difficult to reach portions of the web. For example, many items such as gusseted bags have inwardly extending flaps. Forming a bond or seal between these two inwardly extending flaps is difficult because the sealing mechanism cannot directly engage and heat these inner flaps without also engaging and heating the upper and lower sections of the bag. As a result, the longitudinal seal or seam joining the two free ends of the web is typically located along an easily accessible portion of the web. The longitudinal seal is not located on the inwardly extending flaps of the gusseted bag or other item, even though locating the seam in the gusset area is often desirable for aesthetic or functional reasons.
Another problem with conventional sealing apparatus is that they cannot form contoured seams or seals. The seals extend linearly along the length of the web but cannot vary from this linear path. This limits the usefulness of the sealer for a variety of applications.
The present invention is intended to overcome these and other problems.